Wireless systems are commonly used to transmit information from a first device, such as a base-station, to a second device, such as a key. Moreover, such wireless systems typically involve transmitting information from the second device to the first device, to provide bi-directional communication of information. The second device may then be termed a transponder.
In some applications, the second device may not include its own power supply, and in such applications, it is well known for the second device to obtain the power it requires to operate from external sources by scavenging power or energy. One particular example of such energy scavenging is the use of the energy associated with the transmission of information from the first device, to provide the power for the second device to transmit information back to the first device. Such a system may be termed a field-supplied application. Partly due to the possibility of relatively high powers in the field, such field-supplied applications are particularly prevalent in RFID applications, in which the second device may be, for instance, a passive RFID transponder, or other NFC (near field communication) applications.
In order to efficiently receive power, or to efficiently transmit information, it is important that the front-end circuit of a field supplied application is well-coupled to the field transmitted from and supplied by the base station. It is well-known that the coupling between the field and the antenna coil in the front-end circuit of a transponder is heavily dependent on relative orientation of the antenna coil in the front-end circuit relative to the orientation of the field. In vector terms the coupling depends on the dot product between the two orientations. If the orientations are orthogonal, there will be little or no coupling, and thus the device may not work. In order to prevent this, it is known to include two or three, generally or substantially orthogonal, front-end circuits in the transponder, in order to ensure that at least one circuit is well coupled to the fields. Each front-end circuit may be considered as a separate channel. An immobilizer device includes three orthogonally-oriented antennas configured to receive a signal from a field generated by a base station is disclosed in United Stated patent application publication number US2012/0105219.
In order to utilise the available energy most efficiently, it may be desirable to choose one of the two or three separate channels, and transmit information only on that channel, disabling the other channels. Such a system is disclosed in United States patent application publication number US 2010/0290368, which discloses a half-duplex RFID transponder with an integrated three-dimensional front-end circuit which includes three LC resonant circuits arranged in a three-dimensional configuration. A channel selector is adapted to detect which one of three storage capacitors associated with respective channels, is first charged with a threshold voltage, to select the receiver channel associated to the LC resonant circuit which is coupled to the storage capacitor which is first charged and to deactivate the two other receiver channels.
It would be desirable to provide an alternative method of detection of the most strongly coupled channel, and an associated device.